Field of the Invention
The field of art to which this invention relates includes well tools, and more particularly tools for use in running, setting and testing packoff assemblies in oil or gas wells. United States patents on this type of apparatus can be found in class 166 of the United States Patent Classification System.
Prior to the present invention, various types of devices called packoffs have been used for sealing the annulus between two concentric well elements, such as a wellhead and an inner casing hanger. One well-known type of packoff has an annular resilient seal assembly with a smaller inner diameter and a larger outer diameter than that of the annulus in which it is designed to function, so that when this packoff is set in the annulus an interference seal sufficient to hold the anticipated well pressure is established with the adjacent well elements. Although it has some desirable features, this type of packoff has a serious drawback in that its seal is easily damaged, especially while lowering the packoff through a close-fitting blowout preventer. When such damage occurs the seal must be replaced, since there is no provision for energizing the seal or otherwise overcoming the damage.
Another common type of packoff is set by utilizing the weight of its running string to axially compress the seal element and force it to expand radially into tight contact with the adjacent well elements, and then locking the packoff in this condition with hold-down devices called slips. One of the problems with this "weight-set" packoff is that the slips are difficult release when it is desired to retrieve the packoff from the well. Another problem is that the tapered surfaces of the slips, which establish a friction lock between the packoff and the adjacent well elements, significantly limit the number of locations at which this type of packoff can be set.
A variation of the aforementioned "weight-set" packoff involves the use of hydraulic pressure to produce the downward force needed to compress the seal element sufficiently to establish an adequate pressure barrier. Since this "hydraulic-set" packoff also is locked in place by slips, it is plagued by the same disadvantages of the "weight-set" device.
The most commonly used type of packoff is run into the well on a drill string, and then set by rotating the string to exert torque on the packoff's seal-compressing ring, thereby forcing the seal radially into pressure-tight contact with the adjacent well elements. One of the most desirable advantages of this "torque-set" packoff is that after it has been set in the well the pressure-holding capability of its seal element can be increased by torquing it further, i.e., by further rotation of the drill string. However, when torque-set packoffs are employed at offshore locations where the wellhead is a significant distance below the drilling platform, such as on the ocean floor in deep water, a considerable amount of friction drag exists between the drill string and the walls of the riser, thus requiring the application of excessive torque on the string to fully set the packoff, and incurring the likelihood of serious damage to the string. Furthermore, the vertical movement of the drilling rig on an offshore floating platform makes it very difficult to maintain adequate control of the drill string while setting the packoff, especially during rough weather and heavy seas as, for example, are frequently experienced in the North Sea.
The invention described and claimed in the aforementioned copending Garrett application Ser. No. 650,101 overcomes the foregoing problems and achieves the aforementioned advantages by providing a new apparatus and method for running a torque-set packoff into a well on a drill or other pipe string, and then setting the packoff by torquing it to the desired pressure-holding condition without rotation of the drill string. The apparatus comprises a new type of running and setting tool that interconnects the drill string and the packoff, and that facilitates running, setting, and then pressure-testing the packoff in the absence of any axial load imposed on it by the drill string and/or the tool. The packoff and this running and setting tool are connected in a releasable manner so that the tool can be easily disconnected from the packoff and retrieved from the well for further use.
The Garrett running and setting tool includes a piston that is operated by hydraulic pressure exerted through the drill string, a nut and screw assembly that converts the piston's linear force into torque, and a gear, spline, or other suitable system for transmitting this torque to the packoff. The tool is structured to impose its weight and that of the drill string on the packoff until the packoff is set, and then to transfer this weight to an inner casing hanger or other suitable well element before the packoff is pressure tested, thereby facilitating improved accuracy in measurement of the packoff seal's pressure-holding capability. Should the seal fail to hold the test pressure, the hydraulic pressure in the drill string is increased, thereby causing additional rotation of the packoff's seal-compressing element and further compression of the seal. The Garrett invention also enables this new well tool to be used for running and setting a plurality of packoffs sequentially and at different levels in a wellhead or other suitable outer well element.
In one embodiment of the Garrett invention, the piston of the running and setting tool is attached to the nut of a ball screw assembly, and the lower end of the assembly screw is connected to a gear system that transmits the torque of the screw, which torque is produced by downward movement of the nut in response to hydraulic pressure exerted through the drill string on the piston, to the rotatable seal-compressing element of the padoff. The greater the hydraulic pressure exerted on the piston of the tool, the greater is the amount of rotation of the tool's ball screw, the gear system, and the packoff's seal-compressing element, and thus the greater is the compressive force exerted on the packoff's seal. Accordingly, by means of the Garrett invention the packoff can be set to whatever pressure holding capability is required regardless of the distance thereof from the drilling platform or other location from which the drill string extends, and regardless of the operating conditions present at the surface.